1. Field of the Invention
The invention is directed to a contour collimator for radiation therapy having a prescribed plurality of diaphragm plates displaceably arranged relative to one another. It is particularly directed to a multi-leaf collimator that is utilized for limiting the radiation field of an ionizing radiation, preferably for limiting the radiation field of gamma radiation in a linear accelerator.
2. Description of the Prior Art
Radiation apparatus employed in oncological radiation therapy are equipped with radiation field collimators which only allow the adjustment of rectangularly limited radiation fields. It is currently known, however, that better therapy results could be achieved in many oncological investigations if the radiation dose distribution could be adapted to the usually irregular shape of the target volumes (tumors are usually not spherical).
To this end, irregularly shaped auxiliary collimators are individually fabricated for use in radiation therapy. The equipment required for the manufacture of such collimators is commercially available. Such equipment enables irregular radiation field shapes to be cut out of rigid plates of expanded plastic on the basis of X-ray picture models and to cast the part with metal alloys having a low melting point. This manufacturing procedure can only be carried out given individual irradiation angles and is rather involved.
The use of arbitrarily adjustable collimators based on the multi-leaf principle (what are referred to as "multi-leaf collimators") was proposed by Takahashi as early as 1965 ("Confirmation Radio Therapy", Acta Radiologica, Suppl. 242 (1965), 1-142). Such manually adjustable collimators were then likewise utilized world-wide in various radio therapy centers in the further course of radiation therapy. The advantage over collimators manufactured in accord with the casting principle, however, is slight. Cutting out and casting is replaced by the likewise time-consuming manual adjustment of the individual "collimator leafs" or "diaphragm plates".
The development of motor-adjustable multi-leaf collimators is currently being pursued at many radio therapy centers in view of the availability of inexpensive microelectronic control components. These collimators are provided for employment at neutron irradiation systems, at photon radiation sources and, in particular, at linear accelerators. These unit share the principle of the single-leaf drive. Every leaf (diaphragm plate) of the multi-leaf collimator is driven by its own stepping motor. The number of required stepping motors is identical to the number of individual leaves. The outlay for complicated electronics which is susceptible to malfunction and the space requirement for the integration of such a collimator into an iradiation installation is extremely high since a total of at least forty leaves and, as a result thereof, forty stepping motors, is required.
German patent No. 192 300 discloses a contour collimator wherein two oppositely arranged groups of mutually displaceable, small rods impermeable to X-rays are provided for admitting only a prescribed profile from the radiation field of an X-ray source. This collimator is not suitable for radiation therapy wherein, in particular, high-energy photons (gamma radiation) are employed, since no actual "diaphragm plates" are employed. Moreover, only a manual adjustment of the small rods is provided. Such a manual adjustment, however, is usually too slow for radiation therapy, wherein a plurality of radiation fields having different profiles are successively applied.
German AS No. 1 010 659 discloses a collimator for shaping a useful radiation beam from the radiation of a high-energy radiator, for example a cobalt-60-preparation, comprising diaphragm plates which are adjustable perpendicularly relative to the central ray of the beam to be shaped. In this collimator, a separate adjustment element is provided for every individual diaphragm plate. A drive element, for example, a drive shaft shared by all adjustment elements is connected to each of the adjustment elements only via friction clutches. The limitation of the desired radiation field is prescribed by a perforated plate into which pins are plugged. Given such a collimator, it is difficult to set a new radiation field within a short time. Moreover, the collimator is not suitable for oscillations in a vertical plane. In a certain position, the sliding clutch responds under the influence of the weight of the collimator plates; diaphragm plates would thus fall out, and a change in the contour results. Further, a sliding clutch does not guarantee the patient safety in what is referred to as a one-time irradiation in which the total dose required is applied in fractions as the radiation source and collimator are moved around the patient.
German patent No. 30 30 332 discloses a primary radiation diaphragm for an X-ray examination installation wherein a plurality of gating elements limiting the radiation cone from various sides are employed, these being composed of thin metal strips pressing against one another. The elements are mutually displaceable in the longitudinal direction and are combined in packets. For remotely controllable adjustment, every metal strip carries a nose extending transversely relative to the displacement direction and perpendicularly relative to the gating plane. The nose is disposed at the side of the element facing away from the symmetry axis of the primary radiation diaphragm. Every metal strip packet has an adjustment element allocated to it, this adjustment element being adjustable by an x, y-drive and being engageable with the individual metal strips. This contour collimator is only suitable for low energies since relatively short diaphragm plates are employed. Given a 360.degree. rotation of the collimator around a patient, the individual diaphragm plates would fall out because no interlock is provided. An adjustment element for the individual diaphragm plate is provided, but this can only move low diaphragm plate weights. As a result of its design, moreover, this is limited only to the adjustment of softly shaped contours or profiles, i.e. contours or profiles without steps.